Liquid crystal displays (LCDs) are degraded when subject to a long-term DC potential. A long-term DC potential across pixel electrodes creates an electric field that causes electroplating of ion impurities in the liquid crystal onto the electrodes. Electroplating of the ion impurities creates a residual field on the pixel electrodes that causes image retention on the display.
Drive voltages on an LCD typically have a DC component of approximately zero in order to minimize degradation of the LCD. A pixel is typically driven with alternating drive voltages that provide the RMS voltage value to display an image while maintaining an approximately zero average voltage on the pixel. A pixel will have approximately the same brightness when it is driven at the same magnitude at the opposite polarity.
The four polarity schemes that are typically used to drive a display are frame inversion, line inversion, column inversion, and dot inversion. The pixels in a display are addressed sequentially by rows, beginning with row 1. All of the pixels in a row have a common plate and gate lines.
FIG. 1 illustrates an example of frame inversion. Every pixel in a frame is charged with the same polarity when frame inversion is used. Each pixel is driven with the opposite polarity on the subsequent frame. The polarity is reversed after every change in frame to ensure an average DC potential of zero.
FIG. 2 illustrates an example of line inversion. Adjacent lines on the panel are charged with opposite polarities when line inversion is used. The polarity is reversed before each new frame is scanned to ensure an average DC potential of zero.
FIG. 3 illustrates an example of column inversion. Pixels in adjacent columns are charged with opposite polarities when column inversion is used. The polarities of the pixels in each column in a frame are the same. However, the polarity of each column is reversed in each frame. For example, in Frame N as shown in FIG. 3, columns 1 and 3 are charged with a positive polarity, and columns 2 and 4 are charged with a negative polarity. In the next frame, Frame N+1, columns 1 and 3 are charged with a negative polarity, and columns 2 and 4 are charged with a positive polarity.
FIG. 4 illustrates an example of dot inversion. Adjacent pixels in both the horizontal and vertical directions have opposite polarities when dot inversion is used. The polarity of each pixel is reversed before each new frame is scanned to ensure an average DC potential of zero.
Frame inversion and line inversion can be accomplished with a driving technique known as Common Plate Voltage (Vcom) modulation. Drivers with a low-voltage output range (typically 5V) may be used when Vcom modulation is implemented.
There are three artifacts that can occur on LCDs that can be affected by the polarity scheme: flicker, horizontal cross-talk, and vertical cross-talk. Frame inversion is subject to flicker, horizontal cross-talk, and vertical cross-talk. Line inversion reduces flicker and vertical cross-talk while column inversion reduces flicker and horizontal cross-talk. Dot inversion reduces flicker, horizontal cross-talk, and vertical cross-talk, and results in the highest quality image.
The power dissipation associated with driving an LCD is affected by the polarity inversion scheme being used. The power required to drive the display is proportional to the frequency of polarity reversal of the column line voltages. Frame and column inversion have a polarity reversal frequency equal to the frame rate, while line and dot inversion have a polarity reversal with every line in every frame. Thus, if the LCD has 240 rows, line inversion consumes approximately 240 times as much power as frame inversion.